This invention relates to apparatus for mixing and dispersing gas in the form of fine bubbles in a body of liquid in a tank. The invention can be used in various ways to mix gas with liquid, for example, to add air or ozone to sewage or to remove dissolved oxygen from water by mixing an inert gas with the water to displace the oxygen.
The apparatus also can be used in flotation processes in which suspended solid particles or immiscible liquid droplets are separated from the main body of the liquid. The small bubbles created by the mixing apparatus selectively attach themselves to the particles or droplets to be removed and provide buoyancy to raise them to the surface of the liquid. The material to be separated is taken from the surface of the liquid in the form of a froth. Chemical reagents can be added to the liquid to enhance film forming and bubble adherence to improve separation efficiency. Reagents that induce a froth are called "frothers". Those that assist in the selective separation of one solid from another in a liquid are called "depressers", "deflocculating agents" and "collectors", depending on the specific function formed by the reagent. A good discussion of mixing apparatus on which the present invention is an improvement is in Chemical Engineering, June 8, 1964, pages 165 through 220.
The following U.S. Patents also describe flotation apparatus on which the present invention is an improvement:
U.S. Pat. No. 953,746--Hoover PA1 U.S. Pat. No. 1,976,956--MacLean PA1 U.S. Pat. No. 2,274,658--Booth PA1 U.S. Pat. No. 2,494,602--Wright PA1 U.S. Pat. No. 2,626,052--Carbonnier PA1 U.S. Pat. No. 2,875,897--Booth PA1 U.S. Pat. No. 3,393,802--Logue, et al PA1 U.S. Pat. No. 3,393,803--Daman, et al PA1 U.S. Pat. No. 3,647,069--Bailey PA1 U.S. Pat. No. 3,775,311--Mook, et al PA1 U.S. Pat. No. 3,972,815--O'Cheskey, et al
An exemplary prior art flotation apparatus includes an upright draft tube extending into a body of liquid contained in a flotation cell or tank and an inverted bowl-shaped hood or shroud below the draft tube. An upright rotary shaft extends down the draft tube and rotates an impeller located under the shroud. The action of the impeller forms small bubbles which flow outwardly from under the hood and upwardly through the liquid in the tank.
One disadvantage of this prior art mixing apparatus is that the impeller cannot be set deep enough in large tanks to create sufficient circulation to sweep the bottom clean and still produce the necessary surface flow pattern for the air bubbles to effectively remove material by flotation. This prior art unit also has an undesirable tendency to generate foam which flows in a rotary pattern and stagnates around the draft tube. U.S. Pat. No. 3,972,815 solved these problems of the prior art apparatus by resorting to an elaborate shroud construction having radial vanes and holes in the shroud near the vanes to allow bubbles to circulate upwardly through the holes and radially outwardly toward the edges of the tank. Although highly effective, this mixing apparatus is mechanically complex and consumes substantial quantities of electricity for driving the impeller. Energy consumption can be a substantial factor in the cost of operating such a unit, particularly when a multi-cell unit having a plurality of impellers is employed.